1. Field of the Invention
The present invention relates to an optical waveguide circuit and method of fabricating same.
2. Description of the Prior Art
In the field of modern optical communication, wavelength division multiplexing (WDM) communication has been extensively investigated and developed as means for drastically increasing the transmission capacity. Realization of the WDM communication requires development of an optical waveguide circuit that enjoys high wavelength branching performance or a high isolation characteristic.
FIG. 10 shows an example of an existing optical waveguide circuit. An optical waveguide circuit 20 shown in FIG. 10 is a wavelength multiplexing-branching filter of the Mach-Zehnder type, in which waveguide covres 20b and 20c, including a core and a clad, are formed on a substrate 20a. In the optical waveguide circuit 20, a light beam with wavelengths xcex1 and xcex2 that is projected from an optical waveguide, such as an optical fiber, onto a port P1 of the waveguide core 20b is branched into light beams with wavelengths xcex1 and xcex2 in an optical multiplexing-branching region Ab. The branch beams are guided individually to ports P2 and P3 of the waveguide cores 20b and 20c, and are emitted toward other optical waveguides such as optical fibers that are connected optically to the ports P2 and P3.
In the optical waveguide circuit 20, light incident upon the clad, not upon the core, at the port P1 of the waveguide core 20b and a radiation mode of the light generated in the optical waveguide circuit 20 propagate in the clad as a clad mode. Thus, in a general region Aa behind the optical multiplexing-branching region Ab, the clad-mode light lands on the waveguide cores 20b and 20c and leaks into the ports P2 and P3.
If the clad-mode light is generated in the optical waveguide circuit 20, therefore, the isolation characteristic is lowered to cause cross talks and other adverse effects on the quality of optical communication.
The object of the present invention is to provide an optical waveguide circuit, capable of restraining clad-mode light from landing on a waveguide core or from leaking into other ports, and method of fabricating same.
In order to achieve the above object, according to the present invention, there is provided an optical waveguide circuit having a waveguide core of a desired shape formed on a substrate. In this optical waveguide circuit, light confinement portions having substantially the same refractive index as the waveguide core are arranged along one or both sides of the waveguide core.
In the present specification, the phrase xe2x80x9clight confinement portions having substantially the same refractive index as the waveguide corexe2x80x9d implies that the respective refractive indexes of the light confinement portions and the waveguide core are equal or approximate to each other. Thus, based on the difference in specific refractive index between a core and a clad that constitutes the waveguide core, the width of each light confinement portion is adjusted to a value such that a maximum light confinement effect can be obtained. For example, the width of each light confinement portion is set at about 1 xcexcm or more, preferably at 3 to 20 xcexcm, and most preferably at 5 to 9 xcexcm.
The waveguide core and the light confinement portion are arranged at a distance of 30 xcexcm or more from each other.
Further, the waveguide core has a branching portion at which an optical filter is located across the waveguide core.
In order to achieve the above object, according to the present invention, there is provided a manufacturing method for the optical waveguide circuit having a waveguide core of a desired shape on a substrate and light confinement portions arranged along one or both sides of the waveguide core. In this manufacturing method, the waveguide core and the light confinement portions are formed simultaneously.
According to the present invention, there may be provided an optical waveguide circuit and method of fabricating same, whereby clad-mode light can be restrained from landing on the waveguide core or from leaking into other ports. According to the method of the invention, moreover, the waveguide core and the light confinement portions are formed simultaneously, so that the optical waveguide circuit can be manufactured in simple processes at low cost.
The above and other objects, features, and advantages of the invention will be more apparent from the ensuing detailed description taken in connection with the accompanying drawings.